Americans have plenty of opinions about teenage sexuality. What they don't have are many hard facts. Alfred Kinsey's celebrated sex surveys in the 1940s and '50s were breakthrough studies, but they were methodologically flawed and didn't delve into the issue of teenage sex. And while a major sex study out of the University of Chicago in 1994 did include some teens in its sample, none were younger than 18.

That's why scientists--and concerned parents--were so interested in a new report issued last week by the National Center for Health Statistics. It finally offered solid data about what real kids are doing at home after school, in the back of the car and between the sheets. "We've been dependent on very small studies and anecdotal information when it comes to this type of behavior," says Claire Brindis, a professor of pediatric health policy at the University of California, San Francisco. "This study is truly representative."

It's also huge: government researchers fanned out across the nation in 2002, surveying 12,571 Americans ages 15 to 44. That included plenty of nonteenagers, of course, but it's the teens who provided the most provocative talking points. More than half the adolescents surveyed, for example, said they had engaged in oral sex (and their claims are fairly credible, since the questions were posed not face to face by an adult interviewer but through a specially designed computer program). That proportion was about the same among boys and girls. And although you may assume that girls mostly perform and boys receive, the numbers show the give and take is again about equal.

Another surprise: about 11% of girls 15 to 19 say they have had at least one same-sex encounter--the same percentage that was found in women 18 to 44. (The biggest proportion, 14%, was among women 18 to 29.) You would expect the numbers to rise steadily, if slowly, as women have more opportunities to have such encounters. That they don't suggests that same-sex behavior is more acceptable among teenagers than it used to be.

All those facts are straightforward--but facts need context and interpretation to be meaningful. It's not clear whether younger women are more willing to try same-sex encounters because they're afraid of getting pregnant, because it's trendy or because boys their age can be so crude. ("At least another girl isn't always trying to push her penis into you," said one teenager, reacting to the study.) We simply don't know the answer, says William Mosher, lead author of the study. "That's the kind of thing you could look at with further surveys."

As for oral sex, it's tempting to theorize, as James Wagoner of the Washington-based nonprofit organization Advocates for Youth does, that the abstinence-only movement hasn't prevented sex but has simply pushed teenagers away from intercourse and toward a practice that for some reason they don't think of as real sex. Says Wagoner: "The abstinence-only chickens are coming home to roost."

But in almost the next breath, he admits that there's no way to prove that link or even to say for sure whether oral sex is on the rise. The study, he observes, "is the first baseline that we've had in these areas. It's difficult to identify trends if we've never had confirmed data." What he and others can say for sure, however, is that oral sex carries plenty of dangers of its own, including syphilis, gonorrhea and herpes, as well as papilloma virus, which can cause cervical cancer. "Since we have evidence that kids are engaging in oral sex," says Rachel Jones of the Alan Guttmacher Institute in New York City, "we need to provide them with information about the public-health consequences and how they can avoid them." And that can happen without waiting for any more studies. --With reporting by Carolina A. Miranda/New York